Question

This problem has occurred to me again in the past and I used global variables but I’d like to know what other way there is to deal with it.

The code is from an html5 canvas tutorial I’m playing with at the moment.

var x = 150;
var y = 150;
var dx = 2;
var dy = 4;
var ctx;

function init() {
  ctx = $('#canvas')[0].getContext("2d");
  return setInterval(draw, 10);
}

function draw() {
  ctx.clearRect(0,0,300,300);
  ctx.beginPath();
  ctx.arc(x, y, 10, 0, Math.PI*2, true); 
  ctx.closePath();
  ctx.fill();
  x += dx;
  y += dy;
}

init();

What can I do to avoid using global variables?

Answer 1

You could use the module pattern – it’s a neat way of providing encapsulation:

var app = app || {};
app = (function () {
  var x = 150; // all these variables are private
  var y = 150;
  var dx = 2;
  var dy = 4;
  var ctx;

  var draw = function() { // this method is private
    ctx.clearRect(0,0,300,300);
    ctx.beginPath();
    ctx.arc(x, y, 10, 0, Math.PI*2, true); 
    ctx.closePath();
    ctx.fill();
    x += dx;
    y += dy;
  };

  return {
    init: function() { // this method is public
      ctx = $('#canvas')[0].getContext("2d");
      return setInterval(draw, 10);
    }
  }
}());

app.init(); // init() is public, but all the other vars and functions are not

Read more about it at http://addyosmani.com/largescalejavascript/#modpattern.